A helium balloon is tethered inside a minivan. All the doors and windows of the van are closed.

Question:
The van is:

Choices:
Accelerating forward.
Moving forward at a constant speed.
Accelerating backward.
Moving backward at a constant speed.
Stationary.

 

Suggest change answer “accelerating backward” to “decelerating”. Can’t accelerate backward until forward velocity = 0.

quote:

Originally posted by feersum dreadnaught:
Suggest change answer “accelerating backward” to “decelerating”. Can’t accelerate backward until forward velocity = 0.

How do you mean? "Accelerating backward" is not the same as decelerating. Maybe the van did start at velocity zero (i.e. not moving), then the driver hit the accelerator with the trans in reverse.

Decelerating is not one of the choices.

Having seen that episode....

Has to be accelerating forward.

The car is accelerating forward.

The helium balloon is going to move opposite to the pull of acceleration. Inside the car, the pull of the acceleration is to the rear as the car accelerates. The heavier air surrounding the balloon is "sloshing" to the back of the car under acceleration, forcing the lighter balloon to the front.

Constant speed can’t be told from a still picture. Static charge could hold that balloon in in a less than verticals position also.
What’s funny is the kid in the back has a go pro on his head and the driver is looking at the camera and not the road in front or to the rear.

The balloon is standing still as the car accelerates rearward.

flashguy

Boy sneezed.

Driver has static shoulder while accelerating forward as little boy is pressed back in seat.

There is a very strong fan in that van?

It's explained here: Ask a Physicist

JP

As the car accelerates, the rear surfaces of the car push the air inside the car creating a higher pressure in the back of the car and a lower pressure in the front. The balloon being lighter than air moves forward to the lower pressure area.

That's my guess anyway. It's the same as the seats of the car slamming into you when you hit the gas. It feels like you are being pushed back into the seats, but really your body was stationary and the seats started moving forward with the car.

quote:

Originally posted by senza nome:
There is a very strong fan in that van?

Nothing like that. All the information you need is in the OP.

The van is accelerating, this pushes the mass of air inside to the rear and the balloon moves forward.

Still, but on a steep hill pointing uphill. Background be damned.

Science isn't really my jam. I'd answer, but I think I'd just skew the numbers.
I'm a History major, so I like to work with cold hard facts that make sense...

Insufficient information to formulate an answer. Is AC on in the car, and/or was the balloon affected by the movement of air from the shutting of a door (or doors)?

quote:

Originally posted by bigdeal:
Insufficient information to formulate an answer. Is AC on in the car, and/or was the balloon affected by the movement of air from the shutting of a door (or doors)?

All the information you need to answer the question is in the original post. If something isn't mentioned, it's because it doesn't apply. There is only one best answer.

The car is accelerating forward.

Everything in the car is pushed rearward, including the air inside the car. Helium is less dense than the air around it, and so as the air sloshes rearward, it pushes the balloon up and forward.

As the car stops accelerating and reaches a constant velocity, all of the mass inside the vehicle will stop being pushed rearward as their velocity relative to the car reaches zero. The denser air will settle to the bottom and the balloon will float straight up.